Standardized testing is based on descriptive statistics and norms. Norm-referenced tests compare a student's performance to the performance of other students in a standardized sample group. Reliability ensures tests produce consistent results, while validity confirms a test measures what it claims to measure, such as language versus IQ. Special educators should use norm-referenced tests that closely match their students' groups when possible. Common standardized tests include CELF, OWLS, TOLD, WPT, and ROVPVT/EOVPVT.